1. Field of the Invention
The present invention generally relates to a system and method for managing on the customer side the consumption of power supplied by an electric utility, and more specifically relates to a system and method for reducing the customer's peak power demand charges from an electric utility. Even more specifically, the invention relates to apparatus and a method for sensing stochastic or other peak electrical power demands at a power utility customer and for controlling the operation of a secondary source of electric power situated at the customer side. In addition, the invention relates to apparatus and a method for shifting loads by buying and storing electrical power at the customer side during off-peak power demand periods and using the stored electrical power during high demand times.
2. Description of the Prior Art
Users of large amounts of electric power typically consume such power at uneven rates. This is especially true for customers with highly punctuated loads, that is, having many pieces of equipment with frequent stops and starts. Such customers can be expected to build random and cumulative peaks in their power demands. An example may be the typical supermarket with perhaps four or more different refrigeration systems each with its own independent controls generating elements of the combined peak load.
A typical daily load profile for many industrial manufacturing facilities, retail establishments and the like is made up of several randomly acting loads, such as shown in FIGS. 1A-1D. FIG. 1A illustrates a constant twenty-four hour lighting load. FIG. 1B illustrates a semi-random punctuated load. FIG. 1C illustrates semi-random longer cycle loads, and FIG. 1D illustrates a composite idealized daily load profile, showing high stochastic peaks arising randomly throughout the day.
Despite these fluctuating demands for electric power, electric utilities are required to maintain a generating capacity that exceeds the maximum demand for electricity anticipated during any given period of time. Therefore, electric utilities must maintain generating capacities far in excess of average electrical power requirements in order to meet such occasional and relatively short-term demands. The formation and maintenance of such excess capacity is quite expensive, and dramatically increases the average cost of providing electric power.
In order to better allocate the cost of providing excess power generation capacity to those customers most requiring such capacity, and in order to encourage such customers to distribute their demand for electric power, the utility rate schedule applied to such customers is typically divided into at least two components. The first component is an energy usage charge which reflects the utility's own energy generation and transmission costs. The charge is typically calculated in cents per kilowatt hour of energy consumed during a particular billing period. The second component is a peak demand charge which reflects the utility's capital costs, and is based on the deviation from average energy consumed by the customer during a predetermined demand interval period of time. The peak demand charge is calculated as cents or dollars per kilowatt of actual peak demand. Such peak demand charges can be quite high as a percentage of the total utility charge over a particular billing period.
Owing to the increased use of greater peak demand charges by electric utilities, large industrial consumers of electricity have begun investigating methods for reducing peak power demands from the electric utility. One approach is peak demand shedding. Peak demand shedding is the practice of sequencing equipment use so that only an acceptable number of load contributors are allowed to operate simultaneously. This method is expensive to control and restrictive to the customers.
Another approach is to increase the efficiency of the electrical loads used by the customer. Although this approach is in a positive direction, it yields relatively small gains in reducing peak demands, and it is peak demand that has the greatest effect on the rate that the consumer pays for his power.
A third approach is use peaking generators. Peaking generators are generators provided by the power company to be used during peak demand portions of the day. These may be located in closer proximity to the utility's customer sites. Problems associated with peaking plants (generators) involve high capital investment, low duty cycle, serious environmental considerations and relatively long time before completion.